In general, the non-linear optical effect means a non-linear optical response proportional to the square, cube or higher power of photoelectric field applied. Known examples of the secondary non-linear optical effect proportional to the square of photoelectric field applied include second harmonic generation (SHG), optical rectification, photorefractive effect, Pockels effect, parametric amplification, parametric oscillation, light sum frequency mixing and light difference frequency mixing. Examples of the ternary non-linear optical effect proportional to the cube of photoelectric field applied include third harmonic generation (THG), optical Kerr effect, self-induced refractive index change and two-photon absorption.
As the non-linear optical material of exhibiting these non-linear optical effects, a large number of inorganic materials have been heretofore found. However, inorganic materials can be hardly used in practice because so-called molecular design so as to optimize desired non-linear optical characteristics or various properties necessary for the production of a device is difficult. On the other hand, organic compounds can realize not only optimization of desired non-linear optical characteristics by the molecular design but also control of other various properties and therefore, the probability of its practical use is high. Thus, organic compounds are attracting attention as a promising non-linear optical material.
In recent years, among non-linear optical characteristics of the organic compound, ternary non-linear optical effects, particularly, non-resonant two-photon absorption, are being taken notice of. The two-photon absorption is a phenomenon such that a compound is excited by simultaneously absorbing two photons. In the case where the two-photon absorption occurs in the energy region having no (linear) absorption band of the compound, this is called non-resonant two-photon absorption. In the following, even when not particularly specified, two-photon absorption indicates non-resonant two-photon absorption.
The non-resonant two-photon absorption efficiency is proportional to the square of photoelectric field applied (square-law characteristic of two-photon absorption). Therefore, when a laser is irradiated on a two-dimensional plane, two-photon absorption takes place only in the position having a high electric field strength at the center part of laser spot and utterly no two-photon absorption occurs in the portion having a weak electric field strength in the periphery. On the other hand, in a three-dimensional space, two-photon absorption occurs only in the region having a large electric field strength at the focus where the laser rays are converged through a lens, and two-photon absorption does not take place at all in the off-focus region because the electric field strength is weak. As compared with the linear absorption where excitation occurs in all positions proportionally to the strength of photoelectric field applied, in the non-resonant two-photon absorption, excitation occurs only at one point inside the space by virtue of the square-law characteristic and therefore, the space resolution is remarkably enhanced.
Usually, in the case of inducing non-resonant two-photon absorption, a short pulse laser in the near infrared region having a wavelength longer than the wavelength region where the (linear) absorption band of a compound is present, and not having the absorption of the compound is used in many cases. Since a near infrared ray in a so-called transparent region is used, the excitation light can reach the inside of a sample without being absorbed or scattered and one point inside the sample can be excited with very high space resolution due to the square-law characteristic of non-resonant two-photon absorption.
Therefore, if polymerization can be caused by using excitation energy obtained upon non-resonant two-photon absorption, polymerization can be brought about at an arbitrary position in a three-dimensional space and this enables application to a three-dimensional optical recording medium, a fine three-dimensional stereo-lithography material and the like, which are considered as an ultimate high-density recording medium.
Examples of the technique of performing two-photon photopolymerization with use of a non-resonant two-photon absorbing compound and applying it to stereolithography and the like are described in B. H. Cumpston et al., Nature, Vol. 398, page 51 (1999) [Non-Patent Document 1], K. D. Belfield et al., J. Phys. Org. Chem., Vol. 13, page 837 (2000) [Non-Patent Document 2], C. Li et al., Chem. Phys. Lett., Vol. 340, page 444 (2001) [Non-Patent Document 3], K. D. Belfield et al., J. Am. Chem. Soc., Vol. 122, page 1217 (2000) [Non-Patent Document 4], S. Maruo et al., Oppt. Lett., Vol. 22, page 132 (1997) [Non-Patent Document 5].
However, these techniques have the following problems:
1) the two-photon absorbing cross-sectional area of the two-photon absorbing compound is small,
2) two photons are absorbed directly into a polymerization initiator having a very low two-photon absorbing cross-sectional area, without using a two-photon absorbing compound,
3) a polymerization initiator is not used,
4) the polymerization initiator, if used, has bad matching with the two-photon absorbing compound, and the like. In this way, a high-efficiency two-photon absorbing compound and an appropriate polymerization initiator are not used and this gives rise to problems in practice, that is, the polymerization efficiency is bad and for performing stereolithography or the like by polymerization, a strong laser must be irradiated for a long period of time.
Also, in these publications, a design of causing the photopolymerization composition to undergo modulation of the refractive index between the polymerized area and the non-polymerized area as a result of photopolymerization is not made and not described at all.
On the other hand, an optical information recording medium (optical disc) capable of recording information only once by laser light has been conventionally known and recordable CD (so-called CD-R), recordable DVD (so-called DVD-R) and the like are put into practical use.
For example, in a representative structure of DVD-R, a recording layer comprising a dye is provided on a disc-like substrate where guide grooves (pre-grooves) for tracking the laser light irradiated are formed at a pitch as narrow as a half or less (0.74 to 0.8 μm) of that in CD-R. On the recording layer, a light reflection layer is usually provided and if desired, a protective layer is further provided.
In the recording of information on DVD-R, visible laser light (usually in the range from 630 to 680 nm) is irradiated and absorbed in the irradiated portion of the recording layer, as a result, the temperature is locally elevated to cause physical or chemical changes (for example, production of pits) and in turn, changes in the optical characteristics, whereby the recording of information is effected. In the reading (reproduction) of information, laser light having the same wavelength as the laser light for recording is also irradiated and by detecting the difference in reflectance between the portion of the recording layer where the optical characteristics are changed (recorded area) and the portion where the optical characteristics are not changed (unrecorded area), the information is reproduced. This difference in reflectance is based on so-called “modulation of refractive index” and as the difference in refractive index between the recorded area and the unrecorded area is larger, the ratio of light reflectance, namely, S/N ratio in reproduction is advantageously larger.
Recently, network (e.g., Internet) and high-vision television are rapidly becoming widespread. In addition, the start of HDVT (High Definition Television) broadcasting is near at hand. To cope with such a tendency, demands for a large-capacity recording medium capable of recording image information easily and inexpensively are increasing also in civilian uses.
Furthermore, in business uses such as use for backup of computer or broadcast, an optical recording medium capable of recording large-volume information of about 1 TB or more at high speed and low cost is being demanded.
However, in view of physical principle, conventional two-dimensional optical recording mediums such as DVD-R can have a capacity of about 25 GB at most even if the wavelength of light for recording/reproduction is shortened, and a recording capacity large enough to satisfy the requirement in future cannot be expected.
Under these circumstances, a three-dimensional optical recording medium is abruptly attracting an attention as an ultimate high-density, high-capacity recording medium. In the three-dimensional optical recording medium, recording is superposed in tens or hundreds of layers in the three-dimensional (thickness) direction to achieve super high-density and super high-capacity recording as large as tens or hundreds of times conventional two-dimensional recording mediums. In order to provide a three-dimensional optical recording medium, access and writing must be performed at an arbitrary position in the three-dimensional (thickness) direction and as a technique therefor, a method of using a two-photon absorbing material and a method of using holography (interference) are known.
In the three-dimensional optical recording medium using a two-photon absorbing material, based on the above-described physical principle, so-called bit recording can be performed over tens or hundreds of times and a higher density recording can be attained. Thus, this is very an ultimate high-density high-capacity optical recording medium.
As for the three-dimensional optical recording medium using a two-photon absorbing material, for example, a method of using a fluorescent material for recording and reproduction and reading the information by fluorescence (see, JP-T-2001-524245 (the term “JP-T” as used herein means a “published Japanese translation of a PCT patent application”) by LEVICH, Eugene Boris et al. [Patent Document 1] and JP-T-2000-512061 by PAVEL, Eugen et al. [Patent Document 2]), and a method of using a photochromic compound and reading the information by absorption or fluorescence (JP-T-2001-522119 by KOROTEEV, Nicolai I. et al. [Patent Document 3] and JP-T-2001-508221 by ARSENOV, Vladimir et al. [Patent Document 4]) have been proposed. However, in either method, a specific two-photon absorbing material is not set forth and although examples of the two-photon absorbing compound are abstractly described, the two-photon absorbing compound used has a very small two-photon absorbing efficiency. In addition, these methods have a problem, for example, in the nondestructive reading, the long-term storability of record or the S/N ratio on reproduction and these systems are not practicable as an optical recording medium.
Particularly, in view of nondestructive reading, long-term record storage and the like, it is preferred to use an irreversible material and reproduce the information by detecting the change in reflectance (refractive index). However, a two-photon absorbing material having such a function is not specifically disclosed in any publication.
Also, JP-A-6-28672 by Satoshi Kawada and Yoshimasa Kawada and JP-A-6-1118306 by Satoshi Kawada, Yoshimasa Kawada et al. are disclosing, for example, an apparatus for three-dimensionally recording information by using the modulation of refractive index, and a reproducing apparatus and a reading method for the information, but ε method using a two-photon absorbing polymerizable composition is not specifically described.
As described above, if polymerization can be caused by using the excitation energy obtained upon non-resonant two-photon absorption and as a result thereof, the refractive index can be modulated between the laser-focused area and the unfocused area, modulation of light reflectance due to modulation of refractive index can be brought about at an arbitrary position in a three-dimensional space with very high space resolution and this enables application to a three-dimensional optical recording medium which is considered as an ultimate high-density recording medium. Furthermore, nondestructive reading can be achieved and because of an irreversible material, good storability can be expected, therefore, the practicability is high.
However, since two-photon absorbing compounds usable at present are low in the two-photon absorbing ability, a very high-output laser is necessary as the light source and the recording takes a long time. In particular, for use in a three-dimensional optical recording medium, it is essential to establish a two-photon absorbing three-dimensional optical recording material capable of undergoing modulation of the refractive index by two-photon absorption with high sensitivity. For this purpose, a two-photon absorbing compound capable of absorbing two photons with high efficiency and producing an excited state, and a method of only forming a latent image in the recording by two-photon absorption and causing polymerization due to heat or linear light absorption by using the latent image, thereby effecting the recording, are useful. However, such a material has been heretofore not disclosed at all and establishment thereof is being demanded.